Performance on logical problems involving sequential behavior and on concept identification problems is studied with particular emphasis on measuring on-going problem-solving behaviors during solution. In the logical problems, analysis and synthesis are separate tasks, and age differences and age changes are measured for both. In concept- identification, amount of information gained from each step in the solution is measured, and certain strategies are identified. Memory aspects of the tasks are minimized in some studies, and memory load is a variable in the most recent studies. Some of the research is within the framework of a longitudinal study to provide measures of change in performance with age. Performance of subjects highly experienced on complex complete learning tasks is used to develop individual models of problem solving programable for computers.